EP1373700B1 - Method for purifying exhaust gas of an internal combustion engine - Google Patents

Abstract

The invention relates to a method for purifying exhaust gas of an internal combustion engine, which is operated under lambda control and which has an exhaust gas tract, in which a catalytic converter is located. A pre-catalytic converter lambda value of the exhaust gas is continuously detected upstream from the catalytic converter resulting in the generation of a pre-catalytic converter lambda signal, and this pre-catalytic converter lambda signal is used as a command variable of the lambda control. A post-catalytic converter lambda value of the exhaust gas is continuously detected downstream from the catalytic converter resulting in the generation of a post-catalytic converter lambda signal, which is dependent, in a monotonously decreasing manner, on the lambda value of the exhaust gas downstream from the catalytic converter. In a trim control, a correction of the lambda control is carried out using the post-catalytic converter lambda signal resulting in the generation of a measurement signal. Said measurement signal, at least less than a defined lambda value near lambda being equal to one, is dependent, in a strictly monotonously increasing or decreasing manner, on the lambda value of the exhaust gas downstream from the catalytic converter. In the event that signal levels of the post-catalytic converter lambda signal are greater than a threshold value, the additional measurement signal is used for trim control, and in the event that signal levels of the post-catalytic converter lambda signal are less than this threshold value, the post-catalytic converter lambda signal itself is used for trim control.

Description

The invention relates to a method for purifying the exhaust gas of an internal combustion engine operated by lambda control with an exhaust gas tract in which a catalytic converter is arranged, wherein a pre-catalytic lambda value of the exhaust gas is detected continuously upstream of the catalytic converter, whereby a pre-catalyst lambda signal is generated. the Vorkat lambda signal is used as a reference variable of the lambda control, continuously a Nachkat lambda value of the exhaust gas is detected downstream of the catalyst, and by means of the Nachkat lambda signal in a trim control, a correction of the lambda control is performed.

To clean the exhaust gas is usually located in internal combustion engines that operate on the Otto principle, a three-way catalyst in the exhaust system of the engine. Upstream of this catalyst is a lambda probe which emits a signal which is dependent on the residual oxygen content contained in the exhaust gas. This residual oxygen content in turn depends on the mixture that has been supplied to the internal combustion engine. In the case of excess fuel (rich mixture or air data with lambda <1), the proportion of oxygen in the raw exhaust gas is lower, and in the case of excess air during combustion (lean mixture or air ratios with lambda> 1) higher.

The lambda probes usually used upstream of the catalytic converter, which because of their position are also referred to as pre-catalyst lambda probes, are so-called binary or jump probes. In these, lean output (lambda> 1), the output voltage is usually below 100 mV, increases at a stoichiometric combustion with lambda = 1 almost leaps and reaches in a rich mixture (Lambda <1) Values over 0.6 V; this is called two-point behavior. Characteristic of this two-point behavior of binary lambda probes is that in the region in which the characteristic has a steep slope, therefore, the signal emitted by the lambda probe very much depends on the lambda value of the exhaust gas. For a richer mixture, the slope of the characteristic then flattens significantly from a lambda value close to 1. With currently available binary lambda probes, the resulting kink of the characteristic is approximately at lambda = 0.998.

Lambda probes are also known which provide a clear, strictly monotonically increasing signal in a wide lambda range (between about 0.7 and 4). These lambda probes are referred to as linear lambda probes or broadband lambda probes.

The operation of a lambda-controlled internal combustion engine now takes place so that the lambda value of the raw exhaust gas reproducing output signal of the lambda probe oscillates by a predetermined average, which is assigned approximately lambda = 1. Since a three-way catalytic converter exhibits optimal catalytic properties in the case of the raw exhaust gas with a specific lambda value λ ₀ , this predetermined mean value should actually correspond to λ ₀ . Depending on the catalytic converter, the lambda value λ ₀ , at which there is optimal catalytic action, may differ slightly from lambda = 1, for example at lambda = 0.99, in particular lambda = 0.998.

The dynamic and static properties of each lambda probe are altered by aging and poisoning of the probe. As a result, the position of the signal level corresponding to λ ₀ is shifted. To remedy this, it is known to arrange downstream of the three-way catalyst another lambda probe, the lower thermal stresses due to their greater distance from the engine and due to their location downstream of the catalyst of a lower load is exposed to chemically aggressive substances. This lambda probe, which is also referred to as Nachkat lambda probe due to the position downstream of the catalyst, serves as a monitor probe for monitoring the catalytic conversion and allows fine adjustment of the mixture by the λ ₀ associated signal level of the Vorkat lambda probe is corrected so that the most favorable lambda value λ ₀ for the conversion can always be maintained on average. This method is called guiding or trim control.

From the DE 198 19 461 A1 a trim control method is known in which instead of a Nachkat lambda probe signal from a downstream of a three-way catalyst arranged NO _x -sensitive sensor is used. A similar trim control method using a NO _x sensitive sensor is disclosed in US Pat DE 198 52 244 C1 described.

In the course of the progressive reduction of the pollutants emitted by an internal combustion engine, three-way catalysts are now available, which have a significantly increased conversion rate for hydrocarbons, carbon monoxide and nitrogen oxides. However, it has been found that such highly effective catalysts alter the behavior of the post-catalytic lambda probe in such a way that the slope of the probe characteristic curve in the rich mixture range, ie lambda values <1, is significantly flatter than in the case of brand-new probes or aged probes with conventional ones Three-way catalysts were operated. In addition, the aging usually also leads to a shift in the signal level, ie to a change in the offset, whereby the signal assumes levels in the rich mixture range, which no longer allow reliable evaluation of the signal, since they are outside the manufacturer's specifications. This offset shift additionally aggravates the problem of the curve flattening. With such aged probes a trim control is no longer possible with the required accuracy, or the desired longevity of the Nachkat lambda probe is not achieved.

The invention is therefore based on the object to provide a method for purifying the exhaust gas of an internal combustion engine operated in lambda control, in which with high-efficiency three-way catalysts a trim control with a longer service life of the Nachkat lambda probe is possible.

This object is achieved by a method having the features of the claim. In particular, in an initially described method in which a post-catalytic lambda signal is generated which depends monotonically on the lambda value of the exhaust gas downstream of the catalytic converter, it is achieved by generating a measurement signal which is strictly monotonic at least below a specific lambda value near lambda = 1 rising or falling depends on the lambda value of the exhaust gas downstream of the catalyst, and at signal levels of the Nachkat lambda signal above a threshold, the further measurement signal and at signal levels of the Nachkat lambda signal below this threshold, the Nachkat lambda signal itself is used for trim control.

Thus, according to the invention, the signal of a postcatalyst lambda probe is also used for trim regulation. However, in the lambda range, in which the signal of this probe is no longer suitable for trim control, another generated measurement signal is used for trim control. When this region, in which the signal of the Nachkat lambda probe is no longer sufficiently accurate, is present, it is decided on the basis of the signal level of the Nachkat lambda signal. If this signal level is above a threshold value, the measuring signal is used for trim regulation. If the signal level of the postcatalyst lambda signal is below the threshold value, as is known, the postcatalyst lambda signal is used for trim regulation.

This procedure has the advantage that the trim control on the basis of the conventional post-catalytic lambda signal remains unchanged in the areas in which it continues to show the known good results. Only in the areas where they are due to the highly converting catalyst properties the Nachkat lambda signal is no longer suitable over the entire useful life, this is replaced by the measurement signal.

The requirements for this measurement signal are relatively low. It only needs to be in the area in question, i. then, if the post-catalyst lambda signal is above the threshold, allow a more accurate statement about the lambda value than the post-catalyst lambda signal. This implies that there is an unambiguous assignment between the measurement signal and the lambda value of the exhaust gas downstream of the catalytic converter, which is why the measurement signal must depend strictly monotonically increasing or decreasing on the lambda value.

The threshold value should be such that, at levels of the post-catalytic converter lambda signal below the threshold value, an accuracy of the post-catalytic converter lambda signal is sufficient for the trim control. Since it is no longer the post-catalytic lambda signal that is used above the threshold value for the trim control, it is particularly expedient to select the threshold value such that all signal levels above the threshold value for the trim control no longer allow a sufficient resolution of the lambda value. The threshold thus results from the precision requirements which the trim control places on the post-catalytic converter lambda signal, as well as from the measurement accuracy which the after-catalyst lambda signal can ensure as a function of the lambda value of the exhaust gas.

Due to the two-point course, the probe signal in the range Lambda = 1 has a very large slope. This makes it possible to define the threshold exactly such that it corresponds to lambda = 1. The large slope also ensures a high accuracy of this assignment.

One possible signal suitable as a measuring signal in the invention is the output signal of a broadband lambda probe. Such a broadband lambda probe is therefore advantageous since their characteristic curve over a wide lambda range, in particular over the in the trim control of a stoichiometric mixture operated, lambda-controlled internal combustion engine into consideration, has a relatively constant slope. Switching to the measurement signal of the broadband lambda probe when the postcatalyst lambda probe signal is above the threshold is thus particularly simple.

However, broadband lambda probes have the disadvantage that sometimes a strong shift of the signal level occurs with probe aging. Such behavior, occurring in particular in the case of low-cost broadband lambda probes, has hitherto precluded its use as the sole measuring sensor downstream of a three-way catalytic converter in a trim regulation. According to a preferred refinement of the method according to the invention, it is provided that the threshold value of the postcatalyst lambda signal corresponds to a specific lambda value close to lambda = 1, at the instant at which the postcatalyst lambda signal is equal to the threshold value, the difference between the lambda value indicated by the measurement signal and the determined lambda value is determined and the specific lambda value is determined and this difference is taken into account in the trim control, as far as the measurement signal is used (claim 3).

This ensures that an aging-related change in the signal level, in particular a changed offset, of the measurement signal providing broadband lambda probe is compensated.

If the post-catalytic lambda probe signal of the binary post-catalytic lambda probe reaches the threshold value, then there is an exhaust gas composition with a specific lambda value at this time; So you know at this time the lambda value of the exhaust gas. Due to the knowledge of the lambda value, the measurement signal of the broadband lambda probe with regard to any additive Errors are corrected by the preferred embodiment of the method. Thus, an error adjustment of the measuring signal of the broadband lambda probe takes place at the threshold value.

In the exhaust gas of an internal combustion engine, which is operated with a rich mixture, there is relatively little nitrogen oxide due to the fuel oversupply during combustion, compared with lean combustion, in which excess air exists. It would therefore be expected for a NO _x sensor in the lean range, ie at lambda values <1, no significant dependence of the sensor signal from the lambda value. However, combustion of a rich fuel mixture produces NH ₃ . It is therefore advantageously possible to generate the measurement signal necessary for the invention by means of a NO _x measuring transducer, which exhibits a cross-sensitivity to NH ₃ . This development is particularly advantageous in internal combustion engines having a NO _x -Messaufnehmer, for example, for controlling a NO _x catalyst. In this development, in which the Nachkat lambda signal is obtained by means of a binary lambda probe signal and the measurement signal is obtained by means of a NH ₃ -Quersenslichkeit pointing NO _x probe and below lambda = 1 strictly monotonically decreasing from the lambda value of the exhaust gas depends be used on anyway already provided sensor (claim 4). Additional sensors are not required. By this method, a property of NO _x -Messaufnehmern be positively exploited, which so far in and of itself rather than disturbing and therefore was reduced as possible.

If a binary lambda probe is used to obtain the post-catalytic lambda signal, it is preferable for the threshold value to be 0.45 V (claim 6).

The object underlying the invention is in an alternative embodiment by a method having the features of claim 7 and in particular solved in that by means of a broadband lambda probe, a linear Nachkat lambda signal is generated, which depends strictly monotonically increasing from the lambda value of the exhaust downstream of the catalyst, the linear Nachkat lambda signal is used for trim control and in the presence of a certain signal level the binary Nachkat lambda signal is simultaneously determined an actual signal level of the linear Nachkat lambda signal, from the lambda value, which is assigned to the specific signal level of the binary Nachkat lambda signal, a corresponding desired signal level of the linear Nachkat lambda signal is determined and a difference between Actual signal level and target signal level in the trim control as a correction factor, in particular as an additive factor for offset correction, is taken into account (claim 7).

In this embodiment, the signal of a broadband lambda probe is used continuously for trim control. In order to compensate for age-related shifts in the signal level of such a Nachkat lambda signal, the output signal of a binary Nachkat lambda probe is additionally evaluated to allow already described manner an adjustment of the offset of the Nachkat lambda signal used for the trim control. This solution according to the invention makes it possible to use a post-catalytic lambda signal throughout for the trim control. Switching is not necessary.

The adjustment of the offset can be done intermittently at certain intervals. These should be chosen so that there is no change in the offset between the adjustment times, which could lead to an inadmissible falsification of the trim control.

Fig. 1

eine schematische Blockdarstellung einer Brennkraftmaschine mit einer Abgasreinigungsanlage,

Fig. 2

die Abhängigkeit eines Nachkat-Lambdasignals einer binären Lambda-Sonde sowie eines NO_x-Messsignals eines NO_x-Messaufnehmers vom Lambdawert, und

Fig. 3

die Abhängigkeit eines Nachkat-Lambdasignals einer binären Lambda-Sonde sowie einer Breitband-Lambdasonde.

The invention will be explained in more detail by way of example with reference to the drawing. In the drawing shows:

Fig. 1

a schematic block diagram of an internal combustion engine with an emission control system,

Fig. 2

the dependence of a Nachkat lambda signal of a binary lambda probe and a NO _x measurement signal of a NO _x -Messaufnehmers from the lambda value, and

Fig. 3

the dependence of a Nachkat lambda signal of a binary lambda probe and a broadband lambda probe.

The invention relates to the cleaning of the exhaust gas of an internal combustion engine by means of an exhaust gas purification system, as shown schematically in Fig. 1. It may be a working with Gemischansaugung or direct fuel injection engine. The operation of the internal combustion engine 1 of FIG. 1 is controlled by an operation control unit 2. A fuel supply system 3, which may be formed as an injection system, for example, is controlled by unspecified lines from the operating control unit 2 and worried the fuel allocation for the internal combustion engine 1. In the exhaust tract 4 is a catalyst 5, which has three-way properties. It also has a NO _x -reduzierende function, for the regulation of a NO _x -Messaufnehmer 6 is provided downstream of the catalyst 5. On the NO _x -reductive operation of the emission control system, it does not matter in the following.
The catalyst 5 has due to its three-way properties at a lambda value λ ₀ optimal effect. λ ₀ can be between 0.99 and 1 depending on the catalyst.

For the lambda-controlled operation of the internal combustion engine 1, which is required for optimum three-way effect of the catalyst 5, upstream of the catalyst 5, a pre-lambda sensor 7 is provided which, like the NO _x -Messaufnehmer 6 not their measurements over specified lines to the operation control unit 2 emits. The operating control unit 2 is further supplied with the measured values of further sensors, in particular for the speed, load, catalyst temperature, etc.

With the aid of these measured values, the operating control device 2 controls the operation of the internal combustion engine 1.

The operation of the internal combustion engine 1 takes place in a lambda control so that the signal indicating the oxygen content in the raw exhaust gas signal of the lambda probe 7 in the mean value corresponds to a predetermined signal level. In a normal, fully functional, in particular not subject to aging influences Vorkat lambda probe 7 corresponds to this signal level in the exhaust λ _0, ie the lambda value at which the catalyst 5 has optimal three-way properties.

In order to finely adjust this λ ₀ assigned signal level of the pre-catalyst lambda probe 7 and thus compensate for changes in the pre-catalyst lambda probe, a trim controller 8 provided in the operation control unit 2 checks by means of a post-catalyst lambda signal whose generation is still discussed and which determines the lambda value of the Exhaust gas downstream of the catalyst 5 shows whether the lambda = 1 associated signal level of the pre-catalyst lambda probe 7 is subject to eg age-related displacement. The trim controller 8 then generates a control value that compensates for such a shift, so that it is ensured that the internal combustion engine 1 is controlled by the operating control unit 2 so that the lambda value of the raw exhaust gas in the exhaust tract 4 upstream of the catalyst 5 as closely as possible to the desired lambda value, in which the catalyst 5 has optimal properties, corresponds, therefore, lies in the so-called catalyst window.

The trim controller 8 requires a Nachkat lambda signal for this trim control, which reproduces the lambda value of the exhaust gas downstream of the catalyst 5 with sufficient precision. In the present case, to obtain this signal, a NO _x -Messaufnehmer 6 is used, which emits not only a NO _x -dependent signal but also a binary lambda signal. Of course, a separate binary lambda probe can be found downstream of the catalyst 5 use.

The course of the Nachkat lambda signal as a function of the lambda value is shown in curve 9 of FIG. As can be seen, the output voltage U increases with decreasing lambda values. In the lean region, at lambda values well above 1, the slope of the curve 9 of the post-catalytic lambda signal is relatively flat. In a section 10, which starts at lambda values just above lambda = 1, curve 9, on the other hand, has a very large gradient. At lambda 0.998, at low lambda values, a very low slope section 11 follows. The exact position of the kink formed thereby between the sections 10 and 11 depends on the type of binary lambda probe, but it is regularly close to lambda = 1. The solid line drawn in Fig. 2 curve 9 corresponds to the output signal of a new, binary lambda Probe in conventional three-way catalysts. When used downstream of catalysts that show high static conversion rates, and in particular an increased H ₂ -Anteil in the exhaust stream downstream of the catalyst result, the section 11 runs much flatter on the other hand. This is entered as a dashed section 12 in Fig. 2. Such a flat course of the curve does not permit the exact determination of the lambda value from the postcatalyst lambda signal necessary for the trim control.

Therefore, as soon as the post-catalytic converter lambda signal exceeds the threshold value, for example the value of lambda = 0.998 indicated in FIG. 2, the trim controller 8 no longer uses the postcatalyst lambda signal shown in curve 9, but rather the NO _x concentration indicating signal of the NO _x -Messaufnehmers 6. This signal is shown as a curve 13 in Fig. 2.

Due to a cross-sensitivity to NH ₃ (ammonia), this signal increases below a certain lambda value close to lambda = 1 with decreasing lambda values. In this section 13 of the trimming controller 8 uses the signal of the NO _x transducer for trim control instead of the post-catalytic lambda signal. In the trim control of the trim controller 8 thus switches at a rising signal level of the Nachkat lambda signal from the Nachkat lambda signal to the measurement signal of the NO _x -Messaufnehmers 6 when the signal level of the Nachkat lambda signal above a certain threshold, in this case the lambda = 0.998 corresponding signal level increases.

Instead of the signal of the NO _x measuring transducer 6, a broadband lambda probe can also be used. Their signal is shown in Fig. 3, in turn, the curve 9 of the Nachkat lambda signal is located. The broadband lambda signal 15 depends strictly monotonically increasing from the lambda value. However, it is subject to aging influences, which can lead to a shift by an offset V, so that the broadband lambda signal 15 can also have the characteristic denoted by reference numeral 16. If such aging dependence occurs, the broadband lambda signal 15 is not readily suitable for trim regulation. The trim controller 8 then corrects the offset V in the following way:

If the postcatalyst lambda signal (see curve 9) shows a signal level corresponding to the threshold value (lambda = 0.998 in FIG. 3), the signal level of the broadband lambda signal applied simultaneously is determined. Since the lambda value is known at the same time, the current offset V of the broadband lambda signal can be determined from this. This offset value is taken into account continuously in the determination of the lambda value from the wideband lambda signal 15 if the trim controller 8 uses the wideband lambda signal at signal levels of the postcatalyst lambda signal above the trim control threshold and not the postcatalyst lambda signal.

Alternatively, the broadband lambda signal can also be continuously used for trim regulation, wherein each time the signal level of the postcatalyst lambda signal reaches a predetermined lambda value of the exhaust gas downstream of the catalytic converter 5 indicates the offset V is determined and thereby an adjustment of the wideband lambda signal is achieved.

Claims (7)

1. Method for purifying the exhaust gas from an internal combustion engine (1) which is operated under lambda-based closed loop control and which has an exhaust gas tract (4) in which is located a catalytic converter (5), whereby

   - a pre-converter lambda value for the exhaust gas is continuously sensed upstream of the catalytic converter (5), from which a pre-converter lambda signal is generated,

   - this pre-converter signal is used as the reference variable for the lambda control loop,

   - a lambda value for the exhaust gas is continuously sensed downstream from the converter, from which a post-converter lambda signal (9) is generated, this being a monotonically decreasing function of the lambda value (λ) for the exhaust gas downstream from the catalytic converter (5), and

   - this post-converter lambda signal (9) is used in a trimming control loop (8) to apply a correction to the lambda control loop,

   characterised in that

   - a measurement signal is generated which, at least below a certain value of lambda close to lambda = 1, is a strictly monotonically increasing or monotonically decreasing function of the lambda value for the exhaust gas downstream from the catalytic converter (5), and

   - for levels of the post-converter lambda signal (9) which are above a threshold value this supplementary measurement signal is used for trimming control, and for levels of the post-converter lambda signal (9) which are below this threshold value the post-converter lambda signal itself is used for trimming control.
2. Method in accordance with claim 1, characterised in that

   - the post-converter lambda signal (9) is obtained using a binary lambda probe and

   - the measurement signal is obtained using a broadband lambda probe and on both sides of lambda = 1 is a strictly monotonic increasing function of the lambda value for the exhaust gas.
3. Method in accordance with claim 2, characterised in that

   - the threshold value for the post-converter lambda signal (9) corresponds to a defined lambda value close to lambda = 1,

   - at the point in time at which the post-converter lambda signal is equal to the threshold value, the difference between the lambda value indicated by the measurement signal and the defined lambda value is determined and

   - this difference is taken into account in the trimming controller (8) if the measurement signal is being used by the trimming controller.
4. Method in accordance with claim 1, characterised in that

   - the post-converter lambda signal (9) is obtained using a binary lambda probe and

   - the measurement signal is obtained from an NO_x probe which exhibits cross-sensitivity to NH₃ and below lambda = 1 the signal is a strictly monotonically decreasing function of the lambda value for the exhaust gas.
5. Method in accordance with claim 4, characterised in that the threshold value for the post-converter lambda signal (9) corresponds to that particular value of lambda close to lambda = 1 below which the output signal from the NO_x transducer rises as the value of lambda falls.
6. Method in accordance with one of the above claims, characterised in that the threshold value is 0.45 V.
7. Method for purifying the exhaust gas from an internal combustion engine (1) which is operated under lambda-based closed loop control and which has an exhaust gas tract (4) in which is located a catalytic converter (5), whereby

   - a lambda value for the exhaust gas is continuously sensed upstream of the catalytic converter (5), from which a pre-converter lambda signal is generated,

   - this pre-converter signal is used as the reference variable for the lambda control loop,

   - a post-converter lambda value for the exhaust gas is continuously sensed downstream from the converter by a binary lambda probe, from which a binary post-converter lambda signal (9) is generated, this being a monotonically decreasing function of the lambda value for the exhaust gas downstream from the catalytic converter (5) and having a two-point type graph around lambda = 1, and

   - a trimming controller (8) is used to apply a correction to the lambda control loop,

   characterised in that

   - a broadband lambda probe is used to generate a linear post-converter lambda signal (9) which is a strictly monotonic increasing function of the lambda value of the exhaust gas downstream from the catalytic converter (5) and is used for trimming control,

   - a post-converter lambda value for the exhaust gas is continuously sensed by means of a binary lambda probe downstream from the converter, from which a post-converter lambda signal (9) is generated, this being a monotonically decreasing function of the lambda value for the exhaust gas downstream from the catalytic converter (5) and having a two-point curve around lambda = 1, and

   - if the binary post-converter lambda signal has a defined level an actual signal level for the linear post-converter lambda signal is simultaneously determined, the lambda value which has been assigned to the defined binary post-converter signal level is used to determine a corresponding set level for the linear post-converter signal and the trimming control takes into account any difference between the actual signal level and the set signal level as a correction factor.

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